Power plants fueled by pulverized coal have not had the capability to measure, in real time, the flow of coal to each of the burners in a boiler during normal operation. Current coal flow measurement is determined from tests called clean and dirty air tests. Plant personnel must use pipe penetrations to take these measurements. The objectives of these tests are to calibrate and balance the coal flow to each burner so the plant can operate more efficiently. These tests are done on a periodic schedule and are labor intensive and time consuming. Since direct coal flow measurements have not been possible to date, coal flow has not been used as a control variable. Rather, the amount of coal to the boiler has been indirectly controlled by controlling other variables, such as pressure and temperature, and feeding enough coal to the boiler to maintain these parameters within established setpoints.
The Clean Air Act Amendments of 1990 require all coal fired power plants to limit nitrous oxide emissions to prescribed levels by 1995 and 2000. Owners of these installations must install low nitrous oxide burners. The required new equipment will require more stringent control of the coal flow to the boiler to minimize nitrous oxides and unburned carbon. Unbalanced coal flow to the burners will cause uneven combustion and increase the formation of nitrous oxides. Violation of the emissions regulations will subject the owner of the infracting installation to severe financial penalties. Vendors of low nitrous oxide burners realize the benefits of balanced coal flow to the burners as a means to further reduce the formation of nitrous oxides. These vendors may be able to give significant improvement in nitrous oxide emission guarantees if real time burner balancing were possible.
There are currently no commercial on-line coal flow measurement devices available. One company is testing a microwave coal flow measurement instrument. The microwave measurement technique requires a modification in a section of the coal pipe so the signal can penetrate the pipe wall. This measurement technique also requires constant particle velocity in the pipe, a condition which exists in very few coal fired plants. This microwave measurement technique requires a uniform particle distribution and is not capable of measuring the flow of coal under "roping" conditions. Roping is defined as a closely bunched stream of coal particles that coagulate together in the coal pipe.
Several particle measurement flow meters have been proposed in oil and slurry pipes using acoustic information. For example, one known measuring instrument uses an acoustic cross-correlation technique wherein two acoustic sensors are placed along a pipe section and acoustically decoupled by replacing a section of the pipe with pipe material incapable of transmitting an acoustic signal. The signals from the two acoustically isolated signals are cross correlated and a particle flow is calculated. The disadvantages of this measurement technique are troublesome. First, installation and operation of this instrument would require a modification to each pipe on which it is installed. The modifications necessary to install this equipment at just one power plant would have to be done to coal pipes ranging in number from sixteen (16) to over eighty (80), would be difficult and expensive, and would require a plant outage to make the necessary modifications. Second, an acoustic decoupler is very difficult to install on an eight (8) to twelve (12) inch diameter coal pipe made from carbon steel. It is almost impossible to envision a non-acoustic transmitting material that would also have the structural strength to support the carbon steel pipe interface while serving to decouple the acoustic signal.
Another known technique to measure the mass flow rate of particulate material conveyed hydrodynamically through a pipe by means of a flowing fluid of substantially constant mean velocity derives a flow signal by sensing variations in the capacitance of an electrode exposed to the flowing fluid or air. This method requires a penetration into the pipe or replacement of a section of the pipe with a new section containing the capacitance sensor. Its usefulness in coal burning plants is doubtful, because the velocity, size and quantity of coal particles transported in an air medium create a very abrasive condition under which most, if not all, of the sensors exposed to the particles will be destroyed. The air medium which carries the coal through the pipe in a plant has a variable particle velocity and not the constant mean velocity required by this method.
Another known technique proposes a flow sensor to detect changes in the mass flow per unit time of pneumatically conveyed particles within a system. The sensor is tuned to give an amplitude at a pre-selected mass flow and has as its primary objective the determination of the mass flow deviations from a singular, pre-selected mass flow. This method could not measure coal flow during the full range of plant operations during which the flow continually changes. The non-homogeneous flow characteristic of pneumatically conveyed coal flow and its complexity make this technique impractical in a coal fired plant.
There is a need for a particle flow measurement device capable of measuring directly the flow of coal particles through a pipe without extensive and/or invasive installation.